Document service must validate the request before generating the output

Currently when the request for a specific type of document is received, the API does not validate whether that document type is present in the allowed list of documents for that filing. As a result, there is a very high chance of issuing incorrect documents if the API is accessed directly (not via UI). We need to add a validator to the document service to prevent this.

Cc: @thorwolpert @lmcclung

Start by locating the document service request handler and the allowed list of documents for each filing. Trace how a requested document type reaches output generation, then verify that unsupported types are rejected when the API is called directly while allowed types still generate successfully.

Written by the indexing model from the issue text.